The Great Epic of the Bhārata dynasty 3.187.32

3.187.32

trayo bhāgā hyadharmasya tasminkāle bhavantyuta |
antakāle ca saṃprāpte kālo bhūtvātidāruṇaḥ |
trailokyaṃ nāśayāmyekaḥ kṛtsnaṃ sthāvarajaṅgamam || 32 ||

Therefore, must thou know, O Brahmana that this reward which is obtained by persons having their souls under control and which is unobtainable by the ignorant and the foolish,--this which is attainable by asceticism alone,--is productive of high merit. And, O best of men, at those times when virtue and morality decrease and sin and immorality increase, I create myself in new forms. And, O Muni, when fierce and malicious Daityas and Rakshasas that are incapable of being slain by even the foremost of the gods, are born on earth, I then take my birth in the families of virtuous men, and assuming human body restore tranquillity by exterminating all evils. Moved by my own maya, I create gods and men, and Gandharvas and Rakshasas, and all immobile things and then destroy them all myself (when the time cometh). For the preservation of rectitude and morality I assume a human form, and when the season for action cometh, I again assume forms that are inconceivable. In the Krita age I become white, in the Treta age I become yellow, in the Dwapara I have become red and in the Kali age I become dark in hue, I the Kali age, the proportion of immorality becometh three-fourths, (a fourth only being that of morality). And when the end of the Yuga cometh, assuming the fierce form of Death, alone I destroy all the three worlds with their mobile and immobile existences. With three steps, I cover the whole Universe; I am the Soul of the universe; I am the source of all happiness;